1. Field of the Invention
This invention relates to the gas lift method of obtaining oil from a producing well. More particularly, it relates to effecting corrosion inhibition, scale control, paraffin control and the like during gas lift producing operations. This invention especially relates to the downhole injection of well-treating chemical compositions during gas lift operations to reduce corrosion and to control scale and paraffin wax formation and the like in the well casing, the tubing and their attendant equipment.
2. Description of the Prior Art
Gas lift is a well known technique applied to an oil producing well. The most commonly employed type of gas lift in use today is termed flow valve gas lift because of, as the name implies, the use of special flow valves which make gas lift a self-contained method of production applicable under a very wide range of well conditions. A flow valve is a device which controls the injection of high pressure gas into the fluid contained within the tubing. Flow valves are available in a variety of operating modes. The differential pressure flow valves are spring loaded and remain closed until the pressure in the tubing becomes sufficiently larger, with the help of the spring tension, to overcome the casing pressure causing the valve to open admitting gas from the annulus into the tubing. As the valve opens, the casing pressure is larger than the tubing pressure by the differential of the spring tension. Specific gravity differential flow valves are designed to open as a result of the difference in specific gravity of the fluid in the tubing and a special light fluid contained within the valve. If the tubing fluid is light, the valve remains closed, if it is heavy, the valve opens. Other flow valves may be controlled from the surface by the well operators usually by means of some type of mechanical linkage or by changing the annulus pressure which in turn causes a movement of a pre-pressured metallic bellows connected to the stem of the flow valve. Currently, the surface controlled pressure operated valves are being employed in a majority of gas lift operations.
The flow valve has heretofore been located on the outside of the tubing string. Thus when the tubing is placed in position inside the well casing the flow valve is located in the annulus space. More recently, flow valves have been made retrievable by being located in pockets on the inside of the tubing string. A multiplicity of flow valves may be employed, each at a different level on the tubing to permit variation in the well's production by selective gas injection at a higher or lower positioned valve, as conditions warrant.
Production during gas lift may be either continuous or intermittent. In continuous flow, the well is unloaded to a certain flow valve and at this point equilibrium is established between casing pressure, valve differential, back pressure at the gas injection point, the rate of production and the flow from the reservoir into the well bore. The flow valve remains open, gas is continuously injected into the annulus and the aerated column of fluid is produced at the well head.
Where well conditions will not sustain continuous production, intermittent gas lift is often employed. Gas is introduced into the well casing intermittently, with well fluid accumulating in the tubing between cycles. When the gas is admitted to the tubing through the flow valve, it brings a slug of oil to the surface through the combined actions of displacement and gas expansion.
In oil fields, the production of petroliferous fluids from subterranean formations by means of gas lift is often accompanied by extremely severe corrosion of metal apparatus contacting the fluid produced. In some instances it is found that these fluids contain substantial amounts of organic acid materials such as acetic acid and/or carbon dioxide in the form of carbonic acid. These fluids are classified in the petroleum art as "sweet". In other cases the fluids include corrosive sulfides such as alkali metal and alkaline earth metal sulfides and hydrosulfides, hydrogen sulfide and/or organic sulfides. The sulfide containing fluids are normally designated as "sour". In the absence of some method of retarding the corrosive attack by these sweet and sour fluids, the life of oil well equipment is materially shortened.
A wide variety of chemical corrosion inhibitors has been developed heretofore to reduce the metal corrosion experienced in oil fields. Inhibitors have been developed for the specific corrosion conditions found in a particular field by employing either a single chemical or a mixture of chemicals. Corrosion inhibitors well known in the art are available commercially to combat the corrosion caused by either the sweet or sour fluids and in many instances significant success has been obtained.
A variety of techniques has been employed heretofore to introduce the corrosion inhibitors into the oil production system. It is usually desirable to provide the inhibitor composition in a form readily dispersible in oil. One convenient technique is to form a solution of the inhibitor in oil or other hydrocarbon solvent. The solution may then be injected or poured into the annular space between the casing and the tubing string. Where the inhibitor is a liquid, it may also be employed in its undiluted form if the desired quantity can be accurately measured into the well.
Another method of adding a corrosion inhibitor to a corrosive well fluid is to add the inhibitor in the form of a weighted and disintegratable stick. Such a stick is formed by mixing the inhibitor with an oil soluble or water soluble binder, as the conditions require, in a weighing compound such as barium sulfate or lead oxide. The stick is dropped into the well where solution and emulsification occur.
Metal corrosion can develop during gas lift operations. A number of techniques has been developed heretofore to inhibit this corrosion including squeeze, inhibitor sticks and continuous injection down a small string in the production tubing. Squeeze requires shutting the well in, pumping the inhibitor and carrier downhole and displacing the mixture into the formation. Inhibitor squeeze may cost as much as several days production and often must be done frequently. For highly productive wells such as Berri platforms in Saudi Arabia, a four day yearly shutdown can cost 200,000 barrels of lost production worth about six million dollars per year. The use of inhibitor sticks in the tubing during gas lift is unreliable and slow and thus has not met with a great deal of success. Although continuous injection of inhibitor down a small string in the production tubing provides an accurate means of controlling the introduction of inhibitor into the system, the small string is subject to being pinched, particularly when the string is being pulled since few, if any, well bores are truly vertical.
Other problems closely related to that of metal corrosion may also occur during production operations and may be severe enough to require shutting the well in to take corrective measures. Paraffin wax and scale can form on the metal surfaces of the production tubing, the well casing and other production equipment causing an undesirable reduction in the production rate.
Paraffin wax is often found in petroliferous fluids in subterranean formations. Although this wax is usually soluble under downhole conditions, during production the wax may solidify in a zone of reduced pressure causing undesirable plugging conditions. Liquid solvents have been employed heretofore in production operations to dissolve the solidified wax or to prevent its formation. These techniques are known in the art as paraffin control.
Deposits of inorganic compounds, known as scale, can also take place during production operations, including gas lift operations, causing serious reductions in the production rate. These deposits, which fall out of solution during production and form on the surfaces of the casing, the tubing and other equipment, may comprise such compounds as calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, magnesium hydroxide and the like. Control of this scale by chemical means has been developed over the years and is well known. Inorganic and organic chemicals have been utilized heretofore to obtain a measure of control over this scale formation. These scale control chemicals have been utilized per se if in liquid form. Otherwise it is usually desirable to form a solution or a dispersion of the scale control chemical with a liquid carrier such as oil or other convenient solvent.
Some of the techniques utilized for providing corrosion inhibitors downhole in the production system may likewise be employed with paraffin control solvents and scale control chemicals. Thus, the well can be shut in and the appropriate material, in liquid form or together with a liquid carrier, may be pumped downhole during the delay in production. Production delays whether for paraffin control or scale control are as undesirable and as costly as they are when occasioned by corrosion inhibition measures, as discussed above.
Since corrosion inhibition, paraffin control and scale control can often be effected by the downhole introduction of a liquid, methods for effecting the introduction of one active ingredient are usually equally applicable to the others. Thus, whether a paraffin control solvent, a mixture of a scale control chemical and liquid carrier or a corrosion inhibitor in a carrier is involved, the same equipment and processes may be employed. Hereinafter, the term "liquid, well-treating chemical composition" or a shortened version "well-treating composition" will be used collectively to include a paraffin control solvent, a scale control chemical, a corrosion inhibitor or a like chemical utilized to treat a downhole condition in a production well, whether the chemical is a liquid or is utilized in a liquid form, viz., solution, dispersion and the like.
A need exists for a method of introducing liquid, well-treating chemical compositions at a controllable rate into oil well production equipment during gas lift operations, preferably, without interfering with production and by a method which is not subject to frequent mechanical failure.
It is an object of this invention to provide a method for introducing well-treating compositions into oil well production equipment during gas lift operations.
It is another object of this invention to introduce well-treating compositions into oil wells during gas lift operation without interrupting oil production.
It is a further object of this invention to provide a method of introducing well-treating compositions at a controllable rate into oil well production equipment during gas lift operation.
It is still another object of this invention to provide a method of production well chemical treatment during gas lift operation which is not subject to frequent mechanical failure.